The present standard, A.A.R. approved, ABDW Freight Brake Control Valve manufactured by the Weststinghouse Air Brake Company, includes an accelerated application valve portion that is mounted on the valve emergency portion and operates in response to the quick action chamber "breathing" pressure developed by the emergency piston during service brake applications. Quick action chamber "breathing" is necessary during service brake applications in order to stabilize the emergency piston against undesired emergency operation. This quick action chamber "breathing" pressure is utilized to pilot the accelerated application valve which, in turn, operates to effect a local, quick service reduction of brake pipe pressure in a cyclic manner during service brake applications to thereby supplement the trainline reduction of brake pipe pressure at each car of the train.
It will be appreciated, therefore, that operation of the accelerated application valve is predicated on movement of the emergency piston to a position in which the quick action chamber pressure on one side of the piston is vented to atmosphere, and thus reduced at a rate corresponding to a service rate of reduction of the brake pipe pressure effective on the other side of the piston, thereby preventing the emergency piston from being moved further to an emergency position.
It will be further appreciated that this emergency piston includes a slide valve via which the quick action chamber pressure venting or "breathing" connection is established. In addition to providing the quick action chamber "breathing" function, the emergency piston slide valve also provides for piloting the vent valve to obtain quick action emergency. It is well known that slide valve construction is somewhat more expensive than other valve-type constructions, but a principle advantage is that multiple valve control functions can be incorporated in a relatively small valve device.